Method of using a highly stable aluminum alloy in the production of recrystallization hardened products

ABSTRACT

A METHOD OF USING A HIGHLY STABLE ALUMINUM ALLOY OF THE ALZNMGCU TYPE CONSISTING ESSENTIALLY OF 1.1 TO 1.3% COPPER, 2.3 TO 2.7% MAGNESIUM, 5.7 TO 7.1% ZINC, 0.2 TO 0.5% SILVER, 0 TO 0.09% MANGANESE, 0.02 TO 0.08% CHROMIUM UM, 0.002 TO 0.0006% BORON, AND 0.04 TO 0.08% CHROMIUM IN COMBINATION WITH 0.10 TO 0.16% ZIRCONIUM, AS MATERIAL FOR PRODUCING RECRYSTALLIZATION-HARDENED SEMI-FINISHED CORROSION RESISTANT PRODUCTS BY SUBJECTING THE ALLOY TO A SOLUTION TREATMENT, AND THEREAFTER TO LOW-RATE COOLING.

METHOD OF USING A HIGHLY STABLE ALUlVII- US. Cl. 148-159 6 Claims ABSTRACT OF THE DISCLOSURE A method of using a highly stable aluminum alloy of the .AlZnMgCu type consisting essentially of 1.1 to 1.3% copper, 2.3 to 2.7% magnesium, 5.7 to 7.1% zinc, 0.2 to 0.5% silver, to 0.09% manganese, 0.02 to 0.05% titanium, 0.002 to 0.006% boron, and 0.04 to 0.08% chromium in combination with 0.10 to 0.16% zirconium, as material for producing recrystalli-zation-hardened semi-finished corrosion resistant products by'subjecting the alloy to a solution treatment, and thereafter to low-rate cooling.

The present invention relates to a highly stable aluminum alloy' of the AlZnMgCu type, and to its use in the production of recrystallization hardened, semi-finished materials, products or articles which are resistant to stress corrosion, such as die-pressed components, e.g. castings, or extrusion profiles.

It is an object of thepresent invention to'improve the cross-sectional dependent, recrystallization hardening of such semi-finished materials, products or articles, particularly those of considerable wall thickness, thereby improving the physical properties thereof, or, in those cases in which thesemi-finished materials, products or articles already have satisfactory physical properties, to substantially reduce inherent stresses therein, by moderate cooling thereof after the solution treatment.

Aluminum alloys of the AlZnMgCu type are known to be highly stable. Aluminum alloys of this type used hitherto are preferably produced with a nondeformable and stabilized substructure, which substructure is retained aftersolution treatment and has a molecular structure which is denoted by press effect. This press-effect results from texture, subgrain structure, grainflow in the forming and direction. The use of such alloys in a recrystallized state is, however, also possible provided such molecular structure is obtained during sheet metal production.

y A, large number of aluminum alloys based on AlZnMgCu are known containing varying amounts of each metal; The, most important alloys of this kind are those having substantially the following composition, all proportions being stated as percent by weight: Copper 0 to 3.0%, iron 0 to 0.4%, magnesium 0.75 to 6.0%, silicon 0 to 0.4%, zinc 2.5-to 13.5%, titaniumO to 0.20%, boron 0 to 0.005%, the remainder being'aluminum together with the usual impurities.

. -More particularly, in order to overcome or minimize the problems encountered due to the rupture of alloys as a, result of stress corrosion, favorable results have been "United States Pate 0 ice obtained utilizing alloys containing magnesium and zinc as alloy elements, together, if desired, with the addition of copper in the maximum permissible solubility amount. A known, highly stable AlZnMgCu alloy which, in connection with a two-stage heat hardening process, produces an alloy having satisfactory mechanical and physical properties, has the following preferred composition. Copper 1 to 2%, iron 0 to 0.7%, magnesium 1.2 to 2.9%, manganese 0 to 0.3%, silicon 0 to 0.5%, zinc 5.1 to 6.1%, titanium 0 to 0.2%, zirconium 0 to 0.2%, chromium 0.1 to 0.2%, the remainder being aluminum together with the usual impurities.

The chromium content of such alloy has been found to be of decisive importance, since it helps to combat rupture of the alloy due to stress corrosion and prevents the formation of course-grained recrystallized layers in diepressed components and extrusion profiles. In connection with the separation of E-Al Cr Mg which occurs during the cooling of thick-walled semi-finished products, especially after the solution treatment, by extraction, via the ternary phase, from magnesium, intermetallic hardening is no longer possible in sufficient amounts for the stability increasing zone or the subsequent recrystallization hardening phases 1 or (M); 1 or (M) and T. As a result thereof, there is an increase in the critical cooling speed and a reduction in the physical properties of the resultant product.

Accordingly, therefore, the use of alloy is generally limited to workpieces having a wall thickness of less than mm. Due to the differing stoichiometric recrystallization hardening on the aluminum, the addition of chromium, manganese, vanadium and zirconium mainly intended to prevent the recrystallization phenomena and stress corrosion, tends to act differently on the strength properties of the semi-finished products made of highly stable aluminum alloys and finished by heat treatment. In addition, the morphological effects of such primary separations and the crystallographically conditioned changing cycle of crystal structure faults and separations have to be considered. Including electro-chemical operations, these complex influences determine the static and dynamic characteristics, as well as the characteristic values important in technical designs of fracture tenacity, residual strength and progressive rupture rate of each particular alloy.

With regard to the critical cooling rate, chromium shows the highest negative influence followed -by vanadium and zirconium, provided the amounts of the said elements added is so adjusted that they have the equivalent effect on the prevention of recrystallization. Thus, the following typical individual ingredients can be incorporated in AlZnMgOu alloys, depending upon the solubility thereof in the solid state: Approximately 0.18% chromium, approximately 0.52% manganese, approximately 0.19% vanadium, ap: proximately 0.21% zirconium.

these elements, which prevent recrystallization and improve the resistance to stress corrosion, are associated with different alloy combinations, then the following alloy compositions are obtained:

(1) Zirconium-containing aluminum alloys: 0.5 to 1.8% copper, 0.08 to 0.20% iron, 2.20 to 2.94% magnesium, 0.05 to 0.51% manganese, 0.04 to 0.20% silicon, 5.64 to 6.9% zinc, 0.05 to 0.1% titanium, 0.1 to 0.25% zirconium, 0.05 to 0.5% vanadium.

(2) Zirconiumand chromium-containing aluminum a1- loys: to 1.0% copper, 1.5 to 7% magnesium, 1.5 to 13.5% zinc, 0.05 to 0.5% chromium, 0.05 to 0.5% zirconium.

(3)Zirconium-, chromiumand vanadium-containing aluminum alloys: 0 to 1.0% copper, 1.5 to 7.5% magnesium, 1.5 to 13.5% zinc, 0.05 to 0.5% chromium, 0.05 to 0.5% zirconium, 0.05 to 0.5 vanadium.

A substantial improvement in the so-called stress corrosion resistant alloys is obtained by the addition of silver, particularly if special heat treatments are employed. More especially, the age-hardening relative to silver-free AlZnMgCu alloys is associated with a slight reduction in strength properties.

The following alloys are known:

(1) Zirconium and silver: 0.9 to 1.73% copper, 0.08 to 0.25% iron, 2.12 to 2.67% magnesium, 0 to 0.11 manganese, 0.05 to 0.09% silicon, 5.60 to 6.35% zinc, 0 to 0.01% chromium, 0.03 to 0.4% titanium, 0.28 to 0.35% silver, 0.07 to 0.19% zirconium.

(2) Chromium, manganese, vanadium and silver: 0.1 to 1.5% copper, 0 to 0.4% iron, 1.5 to 6.0% magnesium, 0.1 to 1.5% manganese, 0 to 0.4% silicon, 4 to 12% zinc, 0.1 to 0.06% chromium, 0 to 0.2% titanium, 0.02 to 0.05% boron, 0.1 to 1.0% silver, 0 to 0.15% vanadium.

From this an industrially tested :alloy for forgings and extrusion profiles was developed, preferably of the following manganeseand vanadium-free compositions: 0.9 to 1.2% copper, 0 to 0.25% iron, 2.3 to 2.6% magnesium, 0 to 0.1% manganese, 0 to 0.3% silicon, 5.6 to 6.0% zinc, 0.15 to 0.20% chromium, 0.03 to 0.05% titanium, 0.002 to 0.005% boron, 0.25 to 0.40% silver the remainder being aluminum together with conventional impurities.

It is known, moreover, that AlZnMgCu alloys containing zirconium, as compared with similar alloys containing chromium, have the advantage of a substantially improved penetration hardening. If one compares the highly stable aluminum alloys varyingly modified by the incorporation of chromium and zirconium after the complete heat treatment, inclusive of single or multi-stage heat hardening on workpieces of identical wall thickness, then the zirconium-containing alloys have the advantage over chromium-containing alloys in that they can be treated with very low cooling speeds after the solution treatment in order to obtain the usual strength values. Resulting therefrom are very low, natural stress states which, in turn, mean that during the subsequent processing of the semi-finished products distortion or delay can be avoided.

On the other hand, however, AlZnMgCu alloys which were alloyed with zirconium only, relative to chromiumcontaining alloys, have the disadvantage of a reduction in the resistance relative to stress corrosion. However, the addition of silver to chromium-containing alloys reduces the resistance thereof to stress corrosion accordingly, the favorable results obtainable with the individual use of chromium or a combination of chromium and silver do not extend to an improvement in stress corrosion.

It is an object of the present invention to provide aluminum alloys of the AlZnMgCu type, in which maximum values are obtained for strength, quenching sensitivity, resistance to stress corrosion as well as relatively higher characteristic values of the strengthened alloys.

According to the present invention, there is provided a high strength aluminum alloy of the AlZnMgCu type comprising 1.1 to 3.0% copper, 2.0 to 3.5% magnesium, 5.0 to 7.5 zinc, 0 to 0.4% titanium, 0 to 0.006% boron and 0.04 to 0.1% chromium in combination with 0.08 to 0.3% zirconium, the remainder being aluminum together with the usual impurities.-

The alloys of the present invention are particularly useful for producing hardened semi-finished products which are resistant to stress corrosion, and is applicable to products having both considerable and thin wall thicknesses, which products, after the solution treatment are subjected to low cooling off speeds.

It is, moreover, proposed to keep the manganese content of the alloys of the invention below 0.09%, since any higher proportion leads to high rupture propagation speeds in the material.

The proposal of improving the resistance to stress corrosion of said alloys also appertains to the invention.

For light metal semi-finished products with considerable wall thickness, viz. wall thicknesses substantially in excess of 75 mm., and which are employed, for example, in mining or other Works involving the risk of explosion, it is desirable to restrict the danger of sparking by adding to the alloy in accordance with the invention from 0.004 to 0.02% beryllium.

The invention also includes alloys in which the chromium content is replaced by from 0.05 to 0.20% vanadium. The incorporation of vanadium renders the alloy in accordance with the present invention further insensitive to quenching.

To attain maximum resistance to stress corrosion, the alloys in accordance with the invention are subjected to a two-stage heat treatment, the first stage at a temperature in the region of between and 0., preferably serving a preform of most finely distributed separated material of the type 1 '-MgZn whilst the second heat treatment stage leads to the production of n-MgZnand stable, superhardened T-phase, which acts as nucleus forming agents and can build up on the separated material of the first heat treatment stage. This leads to an improved degree of dispersion and hence also increases the strength values of the final alloy.

The present invention also relates to the further treatment of workpieces made of the said alloys, after solution treatment, to quenching in boiling water, metal melts or molten salts. This moderate cooling ofl after solution treatment presupposes that in the center of the workpieces, a cooling off speed of approximately 2 C./sec. is achieved. Dependent upon the wall thickness of the workpieces, the cooling off speed can be controlled by the temperature of the quenching bath with the object being to attain a natural stress freedom as high as possible, so that a minimum delay is involved in the subsequent shaping of the semi-finished products.

Subsequent investigations on a silver-containing AlZnMgCu alloy, with modified alloy proportions, serve to explain the invention and show the influence of the alloy proportions on the strength properties of solid heat-treated semi-finished products which were produced from cast ingots, the homogenization of which occurred at normal temperatures for such alloy types, viz. between 440 C. and 490 C. The basic alloys then had the following compositions: 5.85 to 6.10% zinc, 2.39 to 2.68% magnesium, 1.10 to 1.15% copper, 0.30 to 0.38% silver, the remainder being aluminum together with the usual impurities. Due to the improved stress corrosion stability thereof, a silver-containing AlZnMgCu alloy was therefore selected. The impurities originating from crude aluminum were 0.08 to 0.13% iron and 0.08 to 0.16% silicon. All charges were refined with a titanium-boronprealloy, so that amounts of titanium of from 0.02 to 0.04% were obtained in the final alloy. The following Table 1 sets out the mechanical properties for two different cooling off speeds, the characteristic values of the active state having been taken into account. The values obtained for the stability to stress corrosion refer to test workpieces of short width. All remaining values bars were utilized as test material:

*6 In such an alloy in the silver-free and silver-containing state, with a cooling olf speed of 5.15 C./sec. (quench- TABLE 5 Rupture Hardening state Mean fracture progressive rate Static strengths tenacity, (quenched in in-mm./load cycle- "fracture 25 0. water) 'f Residual Percent toughness", j H 140 C.-/sec. between am, am, am, strength, 00.2, 013, Km I Alloy, percent by weight 465 and 200 C. 5 kpJrnm. 6 kpJmm. 8 kpJmm. kpJmm. lip/mm. kp./mm. 05 0 Vin.)

AlZnMgCuAg Cr 0.18 std. 160 C. 0.45 1.34 3.85 46.3 57.0 60.5 10 32 43 AlZnMgCuAg Mn 0.88 15 std. 160 C. 1. 81 3. 03 11. 32.0 57. 5 60.9 8 '18 38 AlZnMgCuAg Zr 0.13 15 std. 160 C. 0.85 1. 70 4.36 44. 7 54.7 59. 0 12 27 36 AbZnMgCuAg Cr 0.05 Zr 15 std. 160 C 0. 85 1. 37 4. 43 42. 5 57. 8 62. 1 12 I 30 38 AlZnMgCuAg Cr (105+ 15 std. 160 C 1. 01 2. 15 0. 10 40. 6 53. 5 57. 9 10 21 38 (quenched in boiling water) 50 C./ see. between 465 and AlZnMgCuAg Or 018...... 24 std. 120 C. 0.72 2.21 5.37 39.6 45. 0 51. 1 11.5 15

plus 8 std. 170 C. AlZnMgCuAg Mn 0.88 24 std. 120 C. 2.59 8.30 18.5 28.2 54.6 58.4 7. 5 8

plus 8 std. 170 C. AlZnMgCuAg Zr 0.13-..- 24 std. 120 C. 1.31 2. 39 7. 1 35. 4 56. 5 59. 1 10. 0 AlZnMgCuAg Cr 0.05 Zr 24 std. 120 C- 1. 15 2. 37 6. 3 35. 7 55. 1 58. 1 10. 0 23 0.15. plus 8 std. 170 C. AlZnMgCuAg Cr 0.05 24 std. 120 C. 1.53 2.79 6.5 34.2 52.8 56.5 10.0 24

Mn 0.33. plus 8 std. 170 C.

If the properties are considered according to the static strengths, then the alloy AlZnMgCuAgCrZr, after quenching with water of 25 C., and subjecting to a heat recrystallization hardening of 15 hours at 160 C., enables the comparison of maximum values shown herein to be obtained. The maganese-containing material yields low fracture toughness and relatively low K -values. For these reasons, the use of manganese, either alone or in combination, was not considered further. After quenching in water at 25 and subsequent heat recrystallization hardening, the alloy of the type AlZnMgCuAgCr appears to show maximum results with regard to a compromise with reference to the strength properties exhibited and the resultant steep drop of these properties, after a complete heat treatment with quenching in boiling water, which drop is noticably high. 'It all the properties mentioned are considered then the alloy TABLE 2 Durability in days (quenched in boiling Water) Additional element (percent by weight) Basic alloy The results shown in Table 2, in correlation to Table 1, clearly show the superiority of the combination of chromium and zirconium in such alloys.

The following alloy of the AlZnMgCu with more finely adapted tolerances has proved to be particularly favor able in its strength properties: 1.1 to 1.3% copper, 2.3 to 2.7% magnesium, 5.7 to 7.1% zinc, 0.02 to 0.05% titanium, 0.002 to 0.006% boron, 0.04 to 0.08% chromium in combination with 0.10 to 0.16% zirconium, the remainder being aluminum together with the usual impurities.

hardening from 22 to 35%.

ing in a salt bath at 210 C.) the following favorable strength values were found:

The fracture toughness increased in the time of age- Investigations on silver-free alloys and alloys in which the chromium was replaced by vanadium, showed there to be a reduction in the resistance to stress corrosion, but such reduction still enables such alloys to be satisfactory for technical use. The replacement of chromium by vanadium increased the insensitivity to quenching of the alloy.

In order to obtain maximum strength properties with preferably single stage heat hardening in the region of between and 140 C., 2% of the zinc content of the alloy may be replaced by cadmium. By using the single stage C. hardening, an improvement of the stretch limits and stress corrosion values by an average 2 l p./mm. was observed. Cadmium stabilizes the hardening over G.P.-regions, so that a substantial improvement in the strength values is obtained.

Of particular importance is the smelting of the alloys using repeatedly refined crude aluminum ha-ving limited contents of iron and silicon. Under these conditions the fracture toughness value in the alloy AlZnMgCuAgCrZr increased from 38 to 44. In accordance with the invention therefore, the impurities of iron and silicon should preferably be below 0.1%. For preliminary ingot material with small dimensions, the special refining with titanium and/or boron may be dispensed with, since the nucleus formation occurring due to the titanium compounds in a molten state enables a suiiiciently fine crystalline primary setting to be obtained.

What is claimed is:

1. A method of using a highly stable aluminum alloy of the AlZnMgCu type consisting essentially of 1.1 to 1.3% copper, 2.3 to 2.7% magnesium, 5.7 to 7.1% zinc, 0.2 to 0.5% silver, 0 to 0.09% manganese, 0.02 to 0.05% titanium, 0.002 to 0.006% boron, and 0.04 to 0.08% chromium in combination with 0.10 to 0.16% zirconium, the remainder being aluminum together with the usual impurities, as material for producing recrystallizationhardened semi-finished products which are resistant to stress corrosion, comprising the steps of subjecting the alloy to a solution treatment, and thereafter to low-rate cooling. I

2. The method as recited in claim 1, further comprising the stepof quenching the alloy intboilingwater, t metal melts or in molten salts.

3. The method as recited in claim 1, further comprising the steps of subjecting the alloy after solution treatment to a two-stage recrystallization hardening, ,the first stage being carried out at from 100 to 140 C. for six to thirty hours, and the second stage being carried out at from 160 to 200 C. for two to twelve hours.

4. The method as recited in claim 1, wherein said alloy further comprises between about 0,004 and about I 0.02% beryllium.

5. The method as recited in claim 1, wherein up to 2% of the zinc content of said alloy is replaced by cadmium.

3,198,676 8/1965 Sprowls et a1. 148-159 2,240,940 5/1941 Nock 75 -141 2,823,994 2/1958 Rosenkranz 75-146 3,287,185 11/1966 Vachet et al. 14s 15 9 RICHARD o. EAN, Primary Examiner 7' us; (:1. XR. 

